Exploration of M31 via Black-Hole Slingshots and the "Intergalactic Imperative"
I show that a gravitational slingshot using a stellar-mass black hole (BH) orbiting SgrA* could launch robotic spacecraft toward M31 at $0.1\,c$, a speed that is ultimately limited by the tensile strength of steel and the BH mass, here conservatively estimated as $m_{\rm bh}=5\,M_\odot$. The BH encounter must be accurate to $\lesssim 1\,$km, despite the fact that the BH is dark. Navigation guided by gravitational microlensing can easily achieve this. Deceleration into M31 would rely on a similar engagement (but in reverse) with an orbiting BH near the M31 center. Similarly for a return trip, if necessary. Colonization of M31 planets on 50 Myr timescales is therefore feasible provided that reconstruction of humans, trans-humans, or androids from digital data becomes feasible in the next few Myr. The implications for Fermi's Paradox (FP) are discussed. FP is restated in a more challenging form. The possibility of intergalactic colonization on timescales much shorter than the age of Earth significantly tightens FP. It can thereby impact our approach to astrobiology on few-decade timescales. I suggest using a network of tight white-dwarf-binary "hubs" as the backbone of a $0.002\,c$ intra-Galactic transport system, which would enable complete exploration of the Milky Way (hence full measurement of all non-zero terms in the Drake equation) on 10 Myr timescales. Such a survey would reveal the reality and/or severity of an "intergalactic imperative".